U-100 vs U-40 syringes
The U number is the insulin strength a syringe is printed for. A U-100 syringe puts 100 units in 1 mL; a U-40 syringe puts 40 units in 1 mL. The word "unit" is the same on both, but the volume behind it is not.
What a unit is worth on each
| Syringe | Units per mL | Volume of 1 unit | Volume of 10 units |
|---|---|---|---|
| U-100 | 100 | 0.01 mL | 0.1 mL |
| U-40 | 40 | 0.025 mL | 0.25 mL |
A U-40 unit is 2.5 times the volume of a U-100 unit (0.025 ÷ 0.01 = 2.5). Every calculator on this site, and almost every reconstitution chart, assumes U-100.
What happens when they are swapped
Suppose a calculation for a U-100 syringe gives 20 units. That is 0.2 mL.
- Drawn to the 20 mark on a U-40 syringe, the volume is 20 × 0.025 = 0.5 mL. That is 2.5 times the intended volume, and 2.5 times the amount.
- The opposite swap goes the other way. A figure worked out for a U-40 syringe but drawn on a U-100 gives 0.4 times the intended volume.
Nothing looks wrong when this happens. The plunger sits on the printed number, and the only clue is the label on the syringe.
Same volume on both scales
| Volume | Reads on U-100 | Reads on U-40 |
|---|---|---|
| 0.05 mL | 5 units | 2 units |
| 0.1 mL | 10 units | 4 units |
| 0.2 mL | 20 units | 8 units |
| 0.25 mL | 25 units | 10 units |
| 0.5 mL | 50 units | 20 units |
| 1 mL | 100 units | 40 units |
The table is here to show the size of the gap and to check a suspected mix-up. It is not a suggestion to substitute one syringe for the other. If the syringe you have does not match the one your instructions assume, ask the pharmacy for the right one.
How to tell which syringe you have
- Read the print. The barrel or the packaging states "U-100" or "U-40", often alongside "for use with U-100 insulin only".
- Check the scale against the capacity. On a 1 mL U-100 barrel the scale ends at 100. On a 1 mL U-40 barrel it ends at 40. On a 0.5 mL barrel the ends are 50 and 20.
- Do not rely on cap colour alone. In many markets U-100 syringes have orange caps and U-40 syringes have red caps, but this varies by manufacturer.
Where U-40 syringes come from
Insulin for people is sold almost entirely at U-100 in the United States and most other countries. U-40 is mainly a veterinary strength: several insulins for dogs and cats are 40 units per mL and are dispensed with U-40 syringes. A household with a diabetic pet can have both types in the same drawer. U-40 insulin for people is also still sold in some countries.
U-500 and tuberculin syringes
There is also a U-500 syringe, made for concentrated U-500 insulin. One unit on it is 0.002 mL, a fifth of a U-100 unit. It is uncommon and clearly labelled.
A tuberculin syringe is not an insulin syringe at all. It is marked in mL, typically in 0.01 mL steps on a 1 mL barrel. Its readings convert directly: 0.20 mL is 20 units on a U-100 scale.
Before any calculation
Confirm the syringe is U-100, then use the calculator. If you are holding something else, convert through mL: every tool on this site also shows the volume in mL, and mL means the same thing on every syringe. The syringe units chart lists the scales side by side, how to read a U-100 syringe covers tick marks, and how it works lists the other assumptions behind the arithmetic.
Questions
What is the difference between a U-100 and a U-40 syringe?
A U-100 syringe is marked so that 100 units is 1 mL, so 1 unit is 0.01 mL. A U-40 syringe is marked so that 40 units is 1 mL, so 1 unit is 0.025 mL. A unit on a U-40 syringe is 2.5 times the volume of a unit on a U-100 syringe.
What happens if I use a U-40 syringe with a U-100 calculation?
Drawing to the same number gives 2.5 times the intended volume. For example, 20 units is 0.2 mL on a U-100 syringe but 0.5 mL on a U-40 syringe.
How do I convert U-100 units to U-40 units?
Multiply the U-100 reading by 0.4. The same 0.25 mL reads 25 units on a U-100 syringe and 10 units on a U-40 syringe. This is for checking; use the syringe type your instructions specify.
Are peptide calculators based on U-100 or U-40 syringes?
U-100. This site and nearly all reconstitution charts assume 100 units per mL. If your syringe is not U-100, work in mL instead of units and ask your pharmacist for the correct syringe.